Physical Properties of Gemstones

The physical properties of gemstones, their hardness, their specific gravity or density and they way they break, depend on chemical bonding and the atomic structure within the stone.

Specific Gravity of Density

The specific gravity of a gem is its weight when compared with the same volume of water at a temperature of 4 degrees Celsius. The denser the minerals in the gemstone are, the heavier the weight or specific gravity will be. Heavier gemstones are usually harder as well.

Hardness
Gemstones are often tested by using the Mohs hardness scale to determine just how hard they are. The harder minerals are more durable in that they do not scratch easily and will hold up better in jewelry. Talc is the softest mineral with a hardness of 1 and can be easily scratched with a fingernail. The gemstones with a rating of 7 or over are relatively hard. Quartz gemstones (citrine, amethyst, etc.) range in the 7's, topaz rates 8, and corundum (sapphires and rubies) are a 9 on the Mohs' hardness scale. Diamond registers a 10 and is the hardest known naturally occurring material on earth, more than ten times the hardness of corundum at 9. There is more of a spread between the gems and minerals found between 2 and 3 and between 5 and 6, however corundum is only about 10 per cent harder than topaz.

Cleavage and Fracture

Cleavage is the splitting of gems and minerals along one of the planes related to the stone's structure. Crystalline minerals have cleavage and fracture, whereas amorphous or massive stones only fracture.

Fracture is the way a stone breaks. Consider fracture to be similar to a piece of wood breaking in a direction other than the direction of it's grain. Conchoidal fracture, which is most common in gemstones, shows a series of arcs that spread outward.

When a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture.

Tenacity or Toughness

Tenacity or toughness is the ability of a stone to withstand pressure or impact. Minerals which crumble into small pieces or a powder are said to be brittle. If a gem bends but returns to its original position, it is said to be elastic (mica, nephrite, jadeite); these minerals are tough and difficult to break.

Magnetism & Electricity

Those stones which are attracted by a magnet are considered magnetic, such as magnetite and hematite, which contain iron. Most minerals and gems are poor conductors of electricity. Good natural conductors include native metals and minerals with a metallic luster (pyrite). Natural blue diamond is a semi-conductor.

Thermal Conductivity

Some stones are good conductors of heat, such as quartz, which draws heat away from the body when held and thus feels cold to the touch. A poor thermal conductor, such as amber, feels warm to the touch because it does not conduct heat away from the body. The surface of a genuine gemstone will de-mist more rapidly than that of glass or an artificial stone.

Crystal Systems

Although traditionally crystal systems are not part of the physical properties table of gemstones, but most mineral gemstones are crystalline, with their atoms arranged in regular and symmetrical patterns. Crystal systems are classified into seven different systems, according to the "minimum symmetry" of their faces.